Frequency synthesis is used to generate signals at one or more precise frequencies. These signals may then be used to perform frequency conversion in radio frequency (RF) sensor and communication systems. Frequency synthesis may be provided by several different methods. Of concern in frequency synthesis is the phase and frequency stability of the generated signal. Since the generated signal may be used as a local oscillator signal for frequency up-conversion or down-conversion, instability in the signal results in decreased signal-to-noise performance.
One method of frequency synthesis involves the generation of a multiple tone lightwave signal that can be converted into an RF carrier or local oscillator signal. In this method, optical heterodyning is used to create a sum or difference beat frequency between two optical wavelength tones. The sum or difference beat frequency is then detected by a photodetector or similar apparatus to generate an RF carrier or local oscillator signal. However, the stability of the beat frequency signal is limited by the relative stability of the optical wavelength tones to each other. Of course, the relative stability of the optical tones may be affected by the absolute stability of the tones.
Logan in U.S. Pat. No. 5,379,309, issued Jan. 3, 1995, describes an optical heterodyning apparatus, shown in FIG. 1, that provides a beat frequency signal with improved stability. In FIG. 1, two slave lasers 16, 18 are injection-locked to different optical modes of a master laser 12 to create two phase-coherent laser signals at different frequencies, f1 and f2. The phase-coherent signals are then combined to create a beat frequency at f1-f2. Since the master laser 12 is mode-locked, all of the modes have a well-defined phase relationship that is time-invariant. An RF reference oscillator 10 of a fixed frequency is used to mode lock the master laser. The frequency of the RF reference 10 sets the minimum frequency spacing between the modes. Note that the output of the mode-locked master laser is a periodic series of pulses, which results in the output having a frequency spectrum with multiple frequency tones corresponding to multiple modes. The multi-mode master laser 12 is capable of generating hundreds of locked modes spanning more than 100 GHz. Injection-locking the slave lasers 16, 18 to the master laser 12 provides that each slave laser 16, 18 is tuned to have an output frequency that corresponds to only one mode of the multimode output of the master laser 12. The outputs of the slave lasers 16, 18 are continuous wave signals. Therefore, the apparatus disclosed by Logan is capable of generating a large number of possible RF carrier signals or local oscillator signals over a wide frequency range. The frequency of these signals is selected by tuning the slave lasers 16, 18 to select pairs of maser laser modes with a difference in frequency equal to the desired frequency of the RF carrier signals or local oscillator signals to be produced.
In the apparatus disclosed by Logan, in injection-locking the slave lasers 16, 18 to the master laser 12, the stability of the heterodyne beat signal is degraded by the instability of the laser signals produced by the two injection-locked lasers 16, 18. The heterodyne beat signal has phase noise that is several orders of magnitude worse than the heterodyne beat signal that could be directly produced from two modes of the master laser. This degradation in phase noise performance is due to the dependence of the phase noise performance on the linewidths and phase noise characteristics of the free running slave lasers.
Alternative prior art apparatus comprise multiple slave lasers injection-locked to current modulation sidebands of a master semiconductor laser, as disclosed by K. Kihuchi, C. E. Zah, and T. P. Lee in J. Lightwave Technology, v. 6, n. 12, 1988, pp. 1821-1830. However, only a small number of sideband frequency tones are produced. Thus, only a small number of possible RF carrier signals or local oscillator signals may be generated by optically heterodyning the outputs of the slave lasers.
Other prior art apparatus combine optical injection locking with an optical phase lock loop. See, for example, R. Ramos, et. al, Optics Letters, vol. 19, no. 1, pp. 4-6,1994; A. C. Bordonalli, C. Walton and A. J. Seeds, J. Lightwave Technology, vol. 17, no. 2, pp. 328-342, 1999; L. A. Johansson and A. J. Seeds, IEEE Photonics Technology Letters, vol. 12, no. 6, pp. 690-692, June 2000. FIG. 2 shows a prior art apparatus in which optical injection locking is used with an optical phase-lock loop.
In FIG. 2, a single slave laser 120 is optically injection locked to a single master laser 110. A photodetector 130 is used to detect the difference between the output of the master laser 110 and the output of the slave laser 120 to generate an error signal. The error signal is then directed to a loop filter 140 that controls the current drive of the slave laser 120. FIG. 2 illustrates a configuration of the prior art apparatus with either a homodyne phase-lock loop or a heterodyne phase lock loop. The heterodyne phase-lock loop is provided by the elements shown within the dotted box 101. The elements of the heterodyne phase-lock loop comprise an offset generator 151, a phase detector 153 and a modulator 155. The modulator 155 receives a continuous wave signal from the offset generator 151 that causes the optical signal from the master laser 110 to have an additional frequency tone and the slave laser 120 locks to this additional frequency tone. The phase detector 153 generates the error signal based on the difference between the signal output by the offset generator and the beat signal generated by the difference between the output of the master laser 110 and the output of the slave laser 120. A homodyne phase-lock loop is provided by removing the components of the heterodyne phase-lock loop depicted within the box 101. Essentially, point A is directly connected to point B on FIG. 2 and point C is directly connected to point D on FIG. 2 to provide an apparatus having a homodyne phase-lock loop. With a homodyne phase-lock loop, the outputs of the master laser 110 and the slave laser 120 should be at the same frequency.
The prior art apparatus depicted in FIG. 2 provides that the constraints of the design of the phase-lock loop may be relaxed by using optical injection locking. However, the measured phase noise is still high (greater than −95 dBc/Hz at 10 KHz offset). The phase noise spectrum still is dependent on the linewidths of both the master and slave lasers, although that dependence is reduced by the loop transfer function.
As discussed above, the beat frequency produced at the photodetector 130 when a heterodyne phase-lock loop is used is equal to the frequency of the offset generator 151. If the heterodyne output is to be switchable among various radio frequency or local oscillator frequencies, the offset generator would also need to be switchable among those same frequencies. Prior art apparatus that use switchable or multiple RF reference signal generators to amplitude modulate the light from a single laser without relying upon optical heterodyning and optical injection are known in the art. See, for example, Daniel Yap, et al., “Switched Photonic Link For Distribution of Local-Oscillator Signals,” IEEE Photonics Technology Letters, vol. 12, no. 11, November 2000, pp. 1552-1554. Apparatus that use optical heterodyning and optical injection that also require the use of switchable or multiple RF reference generators provide little advantage over this known art.
Thus, there still exists a need in the art for a switchable frequency synthesizer that can produce a radio frequency carrier or local oscillator signal with low phase noise and minimizes amplitude fluctuations without requiring a switchable RF reference generator or multiple RF reference generators.